1. Stock up on food whenever your low on it so you have less downtime for when your leveling.
2. Always spend however much gold you need to keep your gear updated. If you have gear thats 6 levels lower than you its gonna affect your leveling speed so always have the most up-to-date gear as you possible can.
3. When buying gear the most important attributes you wanna look for are:
-critical strike rating
-haste and agility
these attributes are listed from most wanted to not as much needed, but all attributes are very much needed.
4.The last thing is to spec blood. Blood is the best leveling spec because it has the least downtime out of all the other Death Knight specs and blood has self heals which is very good for leveling.
I hope this has helped any DK's out there!